Some examples relevant to the detection of hydrogen are:
--In case hydrogen leaks occur in closed rooms, such leaks should be quickly detected, in order to prevent any explosion risks. PA1 --Many metals and their alloys (for example, steels, titanium, zirconium, and so forth) react with hydrogen to yield hydrides, with their mechanical characteristics being consequently impaired. PA1 --In many corrosion phenomena, hydrogen generated: therefore, the presence of hydrogen can be indicative of the fact that a corrosion process is taking place (this is useful in particular in order to check parts which are not easily accessible in case of a direct inspection). PA1 --The continuous monitoring of hydrogen in reactions producing or consuming such a gas can be useful in order to control the same reactions for the best. PA1 R is the constant of gases (8.34 J.gmol.sup.-1. K.sup.-1), PA1 F is the Faraday constant (96,500 Coulombs), PA1 T is the absolute temperature as degrees Kelvin and PA1 n is the number of electrons involved in the process.
Prior to 1980, hydrogen was essentially detected by means of polarographic techniques, as disclosed by K. Barthlett, J. V. Dobson, E. Easthman in Chimica Acta 108, 189 (1980), but in the past years a strong trend arose in the art, to replace such a method with simpler and less cumbersome electrochemical devices, such as sensors based on solid-state electrolytes.
In case of hydrogen, such sensors use a protonic-conduction solid, which separates two compartments containing different hydrogen partial pressures.
At the platinum electrodes, installed on both mutually opposite faces of the protonic conductor, an e.m.f. is generated, the value of which is given by the Nernst equation.
For not very high pressures, such an equation can be written as follows: EQU E=RT/nF Ln P.sub.H2 /P.sub.H2(ref) ( 1)
wherein:
In the specific case of hydrogen, n is 2 (in that the process is: EQU H.sub.2 .revreaction.2H.sup.+ +2e).
Going to decimal logarithms, we have, at 25.degree. C.: EQU E=0.0296 log P.sub.H2 /P.sub.H2(ref) ( 2)
If the reference partial pressure is known, the unknown P.sub.H2 pressure can be obtained from a potentiometric measurement of E.
To date, various inorganic solid substances are known, which are endowed with a relatively high protonic conductivity at room temperature, such as, e.g.: uranyl hydrogen phosphate, antimonic acid, phosphomolybdic acid, zirconium hydrogen phosphate and dizirconium triphosphate in hydrogen form.
All these protonic conductors are therefore endowed with applicative potentialities in hydrogen sensors. Also some organic polymers, such as Nafion in hydrogen form, can be used as protonic conductors, such as described by J. Jensen in "Solid State Protonic Conductors for Fuel Cells and Sensors" (page 1, Editors: Godehough J. B. Jensen, Poitier A., Odense University Press, 1985).
Some of such solids have actually given rather good results in the manufacturing of sensors for hydrogen gas (see, e.g., Miura N., Karo N. Yamatoe and Jeiyama T., Proc. Int. Meet. Chem. Sensors, Fukuoka, page 233, 1983).
A further simplification is obtained in the sensor if a reference electrode in the solid state is used, thus a completely solid-state sensor being accomplished.
When a solid-state reference is used, the equation (2) can be advantageously written as: EQU E=constant-0.0296 Log P.sub.H2
wherein the value of the constant depends on the reference electrode used.
For example, a reference in the solid state reported in the relevant technical literature is constituted by a silver foil.